#用Python的lambda来实现凯撒加密算法（右移13位）：
abc = "abcdefghijklmnopqrstuvwxyz"
rt13 = lambda v: "".join([abc[(abc.find(c) + 13) % 26] for c in v])

def test_caesar():
    s = "xthexrussiansxarexcoming"
    assert rt13(s) == 'kgurkehffvnafknerkpbzvat'
    assert rt13(rt13(s)) == 'xthexrussiansxarexcoming'
